Melanie Driscoll, Gulf Coast director of bird conservation for the National Audubon Society, said she's "incredibly dismayed" that oil remains in areas where birds nest and feed in the ongoing breeding season.

Birds can get oil on their feathers and transfer it to eggs or hatchlings, which are especially vulnerable to the oil's toxicity.

With cleanup efforts still under way, the simple act of trying to repair the damage from the oil can actually do more harm.

The small shorebird Wilson's plover is especially susceptible to disruptions, Driscoll said. Cleanup crews have fenced off roads through the grasses that often lie between the birds' nests and their main food sources. Though the birds can squeeze through the fencing, it adds more stress to a bird population that's been declining for decades. (See video: "Citizen 'Scientists' Track Birds in BP-Spill Zone.")

Audubon scientists have also found marine worms burrowing into tar balls in Grand Terre, Louisiana. Lab tests found concentrations of toxic hydrocarbons in the tar balls that could enter the food chain and pose long-term health risks to adult birds or harm developing bird embryos.

Habitat Loss Could Push Birds Over the Edge?

Possibly the most worrying legacy of the spill is the acceleration of habitat loss, scientists say.

For example, some Louisiana bays were hit with heavy oil last summer. The marsh grass that holds the marsh together is dying in many areas—which means that storms, waves, and ship wakes will simply wash away more of the wetlands.

Wildlife & Fisheries' Carloss said it will be difficult to tease out how much habitat loss can be attributed to the spill or the long-standing yearly loss of wetlands from erosion and subsidence. But he says the combination of oiling and cleanup operations in critical marsh habitat clearly must have some effect.

For birds flying thousands of miles from wintering spots in the south to nesting sites as far as the Arctic, any loss of shelter, food, and fresh water could leave them too weak to make the journey or diminish their reproductive success when they reach nesting sites.

"A lot of these birds live right on the edge, with an incredibly physically demanding journey,” said David Ringer, Audubon's communications coordinator for the Mississippi River and Gulf of Mexico.